isl_printer_yaml_start_mapping: handle NULL input
isl_printer_yaml_start_mapping: handle NULL input

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

BenzoSM/isl,BenzoSM/isl,PollyLabs/isl,PollyLabs/isl,tobig/isl,inducer/isl-mirror,UBERTC/isl,inducer/isl-mirror,Meinersbur/isl,Meinersbur/isl,UBERTC/isl,UBERTC/isl,Meinersbur/isl,nicolasvasilache/isl,PollyLabs/isl,tobig/isl,nicolasvasilache/isl,Meinersbur/isl,UBERTC/isl,tobig/isl,BenzoSM/isl,BenzoSM/isl,inducer/isl-mirror,tobig/isl,nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,BenzoSM/isl,nicolasvasilache/isl,inducer/isl-mirror,nicolasvasilache/isl,PollyLabs/isl